The invention is directed to a method for transmitting digital signals in a plurality of transmission channels over an ATM communication network according to an asynchronous transfer mode.
In transmission systems that are operated according to an asynchronous transfer mode (ATM mode), the messages to be transmitted are transmitted in message cells having a fixed length. The message cells each respectively comprise an information part as well as a header part. The information part serves the purpose of accepting the messages, whereas the header part that precedes the information part has information about the allocation of a message cell to a virtual connection. The header part usually has a length of 5 octets, and the information part usually has a length of 48 octets.
A general desire is to provide a universal communication network for voice, text, data and picture (ISDN) on the basis of the asynchronous transfer mode in public and private networks. The problem arises, however, that, for example, the voice information transmitted in the previous transmission technique (pulse code modulation technique PCM) must be converted into message cells and vice versa. Such a conversion must be implemented in multiple fashion under certain circumstances, dependent on the configuration and integration of the ATM communication network. Such a configuration is shown in FIG. 3 by way of example. A uniform ATM transmission network is not yet present therein, but rather a plurality of what are referred to as ATM islands that are integrated in the PCM transmission work. During the course of expansion techniques at a later point in time, these would then probably grow together to form an entire ATM communication network. What this means for the introduction phase, however, is that a plurality of such ATM islands intervene under certain circumstances between a calling A-subscriber and a called B-subscriber. To this extent, thus a multiple conversion of the PCM voice information into ATM message cells and vice versa is necessary.
The conversion of the voice information into message cells is implemented at the interfaces of the PCM transmission network to the ATM communication network. Packeting/depacketing equipment are employed as interface equipment. The job of the PCM network is comprised in integrating the voice information transmitted according to the PCM transmission principle into the information part of the message cell. The job of the ATM network is composed in making the voice information integrated in the information part of the message cells accessible again to the PCM transmission network. If a calling A-subscriber were to exchange voice information with a B-subscriber via such a configuration, then no disadvantages whatsoever would deteriorate the quality of the connection, given a relatively short local call. This changes suddenly, however, with the length of the corresponding connection since a plurality of packeting/depacketing events always involve longer and longer delay times that cannot be tolerated in practice. Unavoidable echo transit times that prove disturbing during a telephone mode arise due to such delay times. For this reason, such technological problems are addressed in CCITT recommendation G131, Blue Book. It is recommended therein that what are referred to as echo suppression techniques are employed beginning with a basic signal transit time of 25 ms. Such techniques, for example, can be the employment of echo suppressors or echo cancelers. In practice, however, the employment of echo suppression systems can no longer represent an economical solution. The aim of the technical field when integrating ATM islands into an existing PCM transmission network is directed in this respect to integrating voice information in message cells in an optimally economical fashion, these message cells then being connected through over an ATM communication network according to the ATM transmission principle. The employment of echo suppressors or echo cancelers is thus avoided at the same time in order to obtain an optimally economical design.
German published application DE 42 18 053 A1 discloses such a method. This method is based on a transmission rate of 64 kbit/s of the voice information in the PCM transmission network. Given the employment of PCM 30 systems, the voice information are accommodated in thirty channels that form a PCM frame together with two further channels that serve the purpose of synchronization and of signalling. The successive PCM frames continue to be defined as a PCM system, whereby a plurality of such PCM systems are employed in practice. The duration of a PCM frame amounts to 125 .mu.s. The information part of a message cell is then filled with voice information from a PCM 30 frame. The complete PCM 30 frame is thereby inserted into the information part of a message cell. The width of a PCM 30 channel amounts to 8 bits. The width of an octet of the information part of a message cell likewise comprises 8 bits. This thus means that the insertion of a complete PCM 30 frame into the information part of the message cell requires 32 octets. Since the information part comprises a total of 48 octets with a width of 8 bits, a total of 16 octets remain free in the method proposed therein. This ultimately means that the message cell is only partially filled. Although such a technique would in fact shorten the packeting time otherwise employed to a considerable extent, the transmission of partially filled cells in practice means that these are correspondingly to be transmitted more often. This in turn produces disadvantages for the traffic load of the overall ATM communication network that cannot be tolerated.